The present invention concerns transmission of data over networks.
The most traditional network is the telephone network. The arrival of ISDN has greatly enhanced the capabilities of telephony and in particular the bandwidth available to users. Telecommunication networks such as telephony, FR (Frame Relay) and x25 are what is know as "connection-oriented." Thus prior to sending information across a connection-oriented network a user must be allocated a circuit either by provision or demand. Once this has been achieved the user can then send and receive information across the network between host machines.
In parallel with the arrival of broadband telephony there has been a very substantial growth in what are known as "connectionless" networks. Connectionless networks encompass LANs but in particular the most significant connectionless network is the Internet.
In a connectionless network a user does not have to obtain a circuit connection with a desired destination prior to sending the information across the network. This is because information is sent in datagrams which contain header field information which intermediate nodes use for routing purposes. There are a number of data networking protocols which have been designed for connectionless network paradigms. These include the Internet Protocol (IP), SNA (Systems Network Architecture), Appletalk and IPX and all transport data in the form of datagrams. Such datagrams will hereinafter be referred to as connectionless datagrams.
In addition to the above two basic types of network, the arrival of broadband networks heralded by the introduction of optical fibres has substantially increased the range and nature of data which can be transmitted over telephone lines. Thus while some users may be content with merely maintaining voice connections at constant bit rates other users might wish to have access to other connection types such as video and data at variable bit rates. Thus users now require the ability to select from a number of connection types in accordance with their needs. What are known as ATM (Asynchronous Transfer Mode) networks have been developed to meet this demand. In a typical ATM network a user can choose between a plurality of potential connection types, for example a fixed rate voice link, a variable bandwidth link peaking at 10 megabits and with a mean of 5 megabits, and a third variable bandwidth link peaking at 20 megabits and having a mean of 8 megabits. The second of these connection types can be used for high speed data transfer and the third for the transmission of video images.
In order to maximise the potential bandwidth available to users the telecommunications industry has developed what are known as virtual path (VP) networks. VP networks differ in two ways from the traditional telephone network already described. In a VP network each user is allocated a bandwidth appropriate to his assumed needs in the various connection types and the ATM network management provide bandwidth by selecting a route from all of the available paths in the network. It will be appreciated that it would be uneconomic to provide each user all the time with all the maximum bandwidth that that user might require. Accordingly, in a VP network the sum of the nominal bandwidths allocated to the users connected in the network is substantially greater than the total bandwidth of the network.
Thus, ATM networks provide a high speed technology that is likely to become fundamental in telephony because of its multi service capabilities. However, ATM still has difficulty in supporting traffic which is based on the connectionless paradigm of the Internet and other LAN-type networks.